In the field of semiconductor and liquid crystal, in fabricating a thin film transistor (TFT), in order to improve the mobility of carriers, a laser annealing technique of forming polycrystalline silicon by applying a laser beam to an amorphous silicon film (hereinafter, referred to as “a-Si film”) formed on a substrate to melt, solidify, and re-crystallize the amorphous silicon film has been performed.
In the laser annealing technique, a pulse laser beam emitted from a laser source is processed into a rectangular beam having a longitudinal section by the use of an optical system and the rectangular beam is applied to the a-Si film on the substrate while relatively moving the rectangular beam relative to the applying surface in a short-side direction of the beam.
An excimer laser is conventionally used as the laser source of the laser annealing technique, but a laser annealing device using a laser beam obtained by converting basic waves of a solid laser such as YAG, YLF, and YVO4 into a visible domain in wavelength has attracted attention (for example, see Patent Documents 1 to 3). The solid laser is useful because it is more advantageous in cost and maintenance than the excimer laser.
In polycrystalline silicon devices or crystallized silicon devices, it is reported that a wavelength-converted beam of a solid laser can be used in processes of activating impurities, etc.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2004-342954    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2004-63924    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2003-347237
In a laser annealing device employing a visible beam of a solid laser, crystals of a semiconductor film are made to grow in the plane direction of the film (referred to as “unidirectional growth”) by uniformizing an energy distribution in the long-side direction of the rectangular laser beam but using the Gaussian energy distribution of the original beam without processing the rectangular laser beam in the short-side direction thereof, thereby obtaining large-diameter crystal grains.
However, since an anisotropic property is given to the crystal grains for the unidirectional growth and a growing distance is affected by deviation in energy every short of a laser pulse, it is difficult to fabricate isotropic and uniform crystal grains. Accordingly, characteristics of transistors are not uniform.
Since the absorption coefficient of a visible beam in a silicon film is low, the energy use efficiency of an incident laser beam is low and much energy is required for crystallization in the laser annealing technique using a visible beam of the solid laser. Accordingly, there is a problem that the processing ability is low.
By applying a laser beam with a low energy density to slow the gradation of the Gaussian energy distribution and thus to suppressing the unidirectional growing distance, it is possible to obtain isotropic and uniform crystal grains by the use of the solid laser. However, the application of a laser beam with a low energy density causes the more decrease in processing ability.